Conventionally, a method of production of .alpha.-oxoaldehyde by oxidative dehydrogenation of alkylene glycol in the presence of a silver catalyst has been known. For example, the following methods are known: a method of obtaining glyoxal at a yield in the 60% order by oxidative dehydrogenation of ethylene glycol, which is the simplest alkylene glycol, with use of metallic silver (silver crystal) as a catalyst, whose particle diameter is in a range of 0.1 mm to 2.5 mm (the Japanese Examined Patent Publication 54011/1986 (Tokukosho 61-54011, Date of Publication: Nov. 20, 1986); and, a method of obtaining glyoxal at a yield of, for example, 82% by oxidative dehydrogenation of ethylene glycol with use, as a catalyst, of metallic silver modified with a phosphorus-containing compound (the Japanese Publication for Laid-Open Patent Application No. 38227/1983 [Tokukaisho 58-38227, Date of Publication: Mar. 5, 1983], the Japanese Publication for Laid-Open Patent Application No. 59933/1983 [Tokukaisho 58-59933, Date of Publication: Apr. 9, 1983], the Chinese Patent No. 85100530 [Date of Publication: Apr. 1, 1985], Catalysis letters, 36(1996) 207-214 [received Jul. 10, 1995, Accepted Oct. 11, 1995]). Also known are: a method of obtaining glyoxal at a yield of 84% by oxidative dehydrogenation of ethylene glycol to which a vaporized phosphorus-containing compound has been added, with use of a granulated metallic silver as a catalyst (the Japanese Publication for Laid-Open Patent Application No.232835/1991 [Tokukaihei 3-232835, Date of Publication: Oct. 16, 1991]); and, a method of obtaining glyoxal at a yield in the 70% order by oxidative dehydrogenation of ethylene glycol with use of, as a catalyst, metallic silver carried by a carrier such as silicon carbide or silicon nitride (The Japanese Examined Patent Publication No.32648/1996 [Tokukohei 8-32648, Date of Publication: Mar. 29, 1996]). In these methods, since glyoxal highly tends to be polymerized, a lot of water (1-3.5 times of ethylene glycol in molarity) is supplied (together with other materials) to a reaction system, so that glyoxal produced is taken out in, for example, a 40-percent-by-weight (wt %) aqueous solution state, as a manufactured product.
The foregoing methods, however, cannot be regarded as methods capable of producing glyoxal at a sufficiently high yield and at a high concentration.
On the other hand, the inventors of the present invention, etc., have proposed, as a method of producing .alpha.-oxocarboxylate from .alpha.-oxoaldehyde, a method wherein .alpha.-oxoaldehyde and alcohol as materials are oxidative-esterified with use of oxygen and a catalyst (the Japanese Publication for Laid-Open Patent Application No.118650/1997 [Tokukaihei 9-118650, Date of Publication: May 6, 1997]). By the foregoing method, by, for example, heating 40 wt % aqueous solution of glyoxal as the simplest .alpha.-oxoaldehyde available in the market, glyoxal in a gaseous state is obtained as a material. To supply vaporized glyoxal to a reaction system, however, sometimes involves difficulties from the viewpoint of industrial application, since the polymerizability of glyoxal is very high. Besides, it follows that glyoxylate as .alpha.-oxocarboxylate is obtained in the presence of water in the reaction system, and hence, glyoxylate is sometimes hydrolyzed thereby, possibly along with other factors, causing the yield to decrease.
In other words, the foregoing conventional methods have a drawback of being incapable of producing .alpha.-oxoaldehyde at a high yield. Further, the same have a drawback of being incapable of stably obtaining a high-concentration .alpha.-oxoaldehyde solution or gas. Moreover, because of these drawbacks involved in the foregoing methods, there further arises a problem that it is impossible to produce .alpha.-oxocarboxylate at a high yield.
The present invention has been made in light of the foregoing conventional problems, and an object of the present invention is to provide a method wherein .alpha.-oxoaldehyde, at a higher concentration than conventionally, in a solution state or in a gaseous state, can be stably produced at a higher yield than conventionally. Another object of the present invention is to provide a method wherein .alpha.-oxocarboxylate is produced at a higher yield than conventionally.